This is actually a very good question! There are a couple of ways you can approach it:

• You can begin to incorporate an email sign up list into the signature of all your articles and active readers of yours will begin to join.
• You cannot directly message all of your personal followers but if you create a publication you can directly reach out to your publication followers (although you still won’t have their emails).
• You could try to reach out to key followers on channels outside of medium such as through Facebook groups or other chats.

Generally though, it will take a while to convert even a small percentage of your medium followers into email subscribers, which is why it’s best to start collecting emails as soon as you can.